Heretofore, as shown in Japanese laid-open patent publication No. 8-189,485 (hereinafter referred to as a patent publication 1), there has been known a screw machine for drawing and discharging a fluid by rotating a pair of screw rotors synchronously in opposite directions in a non-contact manner with the use of timing gears. In the patent publication 1, there has been proposed an example of a tooth profile of the screw rotor which can be easily designed and manufactured. Specifically, the respective screw rotors have teeth having the same shape as each other and extending helically in opposite directions. The tooth profile along an axial direction of the screw rotor (hereinafter referred to as an axial tooth profile) comprises an outer circumferential section and a tooth root section which extend straightly with the same length and are located apart from a pitch line by an equal distance, and two curves interconnecting the outer circumferential section and the tooth root section. One of the two curves has point symmetry with respect to a pitch point where the above curve crosses the pitch line, and interconnects the outer circumferential section and the tooth root section smoothly. The other curve is defined by a trochoid curve generated by a point on an outer circumferential surface of the companion screw rotor in a transverse (normal-to-axis) cross section of the screw rotor.
In Japanese laid-open patent publication No. 9-324,780 (hereinafter referred to as a patent publication 2), there has been proposed a screw machine having two pairs of screw rotors which are fixed to two rotating shafts and are driven synchronously in opposite directions by a two-axis synchronous brushless DC motor without using the timing gears.
The screw machine disclosed in the patent publication 1 requires the timing gears for rotating the screw rotors synchronously in the opposite directions in a non-contact manner. On the other hand, in the screw machine disclosed in the patent publication 2, the screw rotors can be rotated synchronously in the opposite directions without using the timing gears because of magnetic coupling of the two-axis synchronous brushless DC motor. However, in both cases, the screw rotors are brought into contact with each other due to disturbance such as sudden fluctuation of a discharge pressure of the screw machine. In such a case, with the conventional tooth profile, the screw rotors are brought into contact with each other not only at the pitch line but also at other points. Accordingly, contact portions of the screw rotors are moved relative to each other, there by causing heat generation and wear of the contact portions. Further, the screw rotors are brought into intense contact with each other to cause the rotations of the screw rotors to be restrained. Therefore, in the case where the magnetic coupling is used to rotate the screw rotors synchronously in the opposite directions, a clearance between the screw rotors is required to be larger compared to the case where the timing gears are used, thus causing a performance of the screw machine to be lowered.